Launch.json and tasks.json currently support several config variables (https://code.visualstudio.com/docs/editor/tasks#_variable-substitution)
However I could not find the ability to know the currently selected line in the current file.
Adding this variable will allow such tasks as running the test under cursor with external command without writing a full blown extension for it.

As I understood the file responsible for adding config variables is the following: https://github.com/Microsoft/vscode/blob/62d4b783965fcabc431c786146917593a2772b3d/src/vs/workbench/services/configurationResolver/node/configurationResolverService.ts#L77
I could also find a code from vscode-go that allows running test under the cursor:
https://github.com/Microsoft/vscode-go/blob/0e5a97fa5cfd4e3b37e5b2fbfbf9c788f0381878/src/goTest.ts#L44